This invention relates generally to a way to improve a fuel cell humidifier unit, and more particularly to the construction of a water-permeable membrane within the humidifier and a method for making the same.
In many fuel cell systems, hydrogen or a hydrogen-rich gas is supplied through a flowfield to the anode side of a fuel cell while oxygen (such as in the form of atmospheric oxygen) is supplied through a separate flowfield to the cathode side of the fuel cell. An appropriate catalyst (for example, platinum) is typically disposed as a layer on porous diffusion media that is typically made from a carbon fabric or paper such that the combination exhibits resiliency, electrical conductivity, and gas permeability. The catalyzed diffusion media is used to facilitate hydrogen oxidation at the anode side and oxygen reduction at the cathode side. An electric current produced by the dissociation of the hydrogen at the anode is passed from the catalyzed portion of the diffusion media and through a separate circuit such that it can be the source of useful work, while the ionized hydrogen passes through another medium situated between the diffusion media of the anode and cathode. Upon such passage, the ionized hydrogen combines with ionized oxygen at the cathode to form high temperature water vapor as a reaction byproduct. In one form of fuel cell, called the proton exchange membrane or polymer electrolyte membrane (in either event, PEM) fuel cell, the medium for ionized hydrogen passage is an electrolyte in the form of a perfluorinated sulfonic acid (PFSA) ionomer membrane (such as Nafion®). This layered structure of the membrane surrounded on opposing sides by the catalyzed diffusion media is commonly referred to as a membrane electrode assembly (MEA), and forms a single fuel cell. Many such single cells can be combined to form a fuel cell stack, increasing the power output thereof.
Fuel cells, particularly PEM fuel cells, require balanced water levels to ensure proper operation. For example, it is important to avoid having too much water in the fuel cell, which can result in the flooding or related blockage of the reactant flowfield channels. On the other hand, too little hydration limits the electrical conductivity of the membrane and can lead to premature cell failure. Increasing the difficulty in maintaining a balance in water level is that there are numerous conflicting reactions taking place in a fuel cell that are simultaneously increasing and decreasing local and global hydration levels.
One potential method of ensuring adequate levels of hydration throughout the fuel cell includes humidifying one or both of the reactants before they enter the fuel cell. For example, residual water present at the cathode exhaust can be used with an appropriate humidification device to reduce the likelihood of dehydration of the anode, the PFSA ionomer membrane, or the cathode inlet. One such humidification device is a water vapor transfer (WVT) unit, which may also be referred to as a membrane humidifier, fuel cell humidifier, or related assembly. The WVT unit extracts the moisture from a humid fuel cell exhaust flowpath and places it into the feed path of a reactant low in humidity. Wet-side and dry-side reactant flowpaths (for example, a cathode exhaust and a cathode inlet) are in moisture-exchange communication with one another in the WVT unit through one or more separators (also known as separator plates). In a particular manufacturing approach, the separator is formed continuously as a roll with a pair of planar porous layers and a support spaced apart by elongated strings placed between them. From this continuous roll, the WVT unit can be cut into sizes and shapes needed for a particular fuel cell application. Examples of WVT units may be found in U.S. Pat. Nos. 7,749,661 and 7,875,396, as well as US Published Patent Application 2009/0092863, all of which are assigned to the assignee of the present invention and the entire contents of which are herein incorporated fully by reference.
The exchange of humidity is generally accomplished in the WVT unit by using an ionomer membrane disposed between adjacent high humidity and low humidity fluid flowpaths formed in the separators. The generally planar membrane (which may structurally resemble that of the PFSA membrane discussed above) allows water vapor to pass from the higher humidity fluid on one side to the lower humidity fluid on the other while inhibiting the direct mixing of the two fluids, for example, the cathode inlet and cathode exhaust that are being conveyed through the flowpaths. In one form of construction, the ionomer membrane is attached to an adjacent support layer (which may be a thin layer of expanded poly(tetrafluoroethylene) (ePTFE) or related material) to increase the robustness and handleability of the membrane and support, as well as to avoid migration of the ionomer into a porous separator. Despite such layering, the combination remains fragile where, in one typical form, the ionomer membrane may be between 3 and 10 microns thick, while the support (made, for example, from expanded poly(tetrafluoroethylene) (ePTFE)) may be about 10 to 30 microns thick.
Existing processes associated with WVT unit fabrication may expose the ionomer membrane and support to excessive handling and related harsh conditions that could jeopardize membrane integrity through failure modes such as crossover. Likewise, existing processes that end up scrapping significant portions of ionomer membrane result in waste and related cost increases. As stated above, the separator is produced in roll form; in such form, the ionomer membrane is on a supporting layer of backer material which must be removed as part of the separator plate manufacturing process. These features significantly contribute to the overall cost of incorporating ionomer membranes into the WVT unit. As such, approaches that would reduce both the handling and cost issues associated with such membrane material are preferred.